1. Fields of the Invention
The present invention relates to a data transmitting-receiving system, more particularly, to a system in which data is transmitted by application of digital modulation to a carrier in a transmitter and in which the thus modulated data is demodulated in a receiver.
2. Description of the Prior Art
The usual methods readily available for commercial use for digital modulation are phase shift keying (PSK), quadrature amplitude modulation (QAM), and so on. These methods have made it possible to achieve high speed transmission of enormously large volumes of data.
In these methods, the carrier is modulated in the phase component and/or amplitude component thereof by the data to be transmitted. Considered schematically, therefore, the coded points corresponding to data may be thought of as being distributed on a plane. There are, however, many phases for modulation and/or many amplitudes for modulation in these modulation methods. This makes the modulator and demodulator complicated in design. These modulation methods are therefore not so preferable from the technical and economic standpoints.
In prior art frequency division multiplex radio communication systems, it has been known to alternate the channels between vertically (V) polarized waves and horizontally (H) polarized waves to obtain close channel allocation to make effective use of the frequency band. Thus, to increase data transmission capacity, it should be possible to allocate channels having V-polarized waves and H-polarized waves as their carriers in the same frequency area. In practice, however, simply setting channels using V- and H-polarized waves in the same frequency area is not sufficient for realization of a practical radio communication system since undesired interference occurs between the V- and H-polarized wave channels due to reduced cross polarization discrimination.